1. Field of the Invention
The present invention relates to a fixing device which is employed in an image forming apparatus such as a printer, a copying machine, a facsimile, and adapted to heat and melt an unfixed toner image which is carried on a recording medium, thereby to fix it on the recording medium.
2. Description of the Related Art
Generally, as a process for fixing a toner image in an image forming apparatus using powdery toner, there has been widely adopted such a method that the toner image is transferred onto a recording medium electrostatically, or the toner image is secondarily transferred to the recording medium after it has been primarily transferred to an intermediate transfer body, thereafter, the recording medium is interposed between a heating member and a pressurizing member, and then, the toner image is heated thereby to be fixed on the recording medium under pressure.
One of the conventional fixing devices includes, for example, a fixing roll having a heating body such as a halogen lamp which is contained in a cylindrical core metal, and a pressure roll to be pressed with this fixing roll. A recording medium carrying an unfixed toner image thereon is interposed between the fixing roll and the pressure roll, and then, heated and pressurized.
The fixing device as described above lacks in quick starting performance, because it is difficult to decrease thermal capacity of the fixing roll, and a considerable waiting time is required until the fixing roll and so on are heated up from a completely cooled state to a determined fixable temperature, even though the halogen lamp which is a heat source for the fixing device is energized simultaneously when the image forming apparatus is switched on. Moreover, under waiting condition of the image forming apparatus (when an image is not outputted), the halogen lamp must be always energized to keep the fixing roll at a determined temperature so that image forming operation can be conducted any time, and therefore, high consumption of electric power is inevitable.
On the other hand, there has been such a fixing device in which an endless fixing belt is employed in place of the fixing roll. A type of the fixing belt is stretched by means of a plurality of support rolls, and another type of the fixing belt has a pressure member inside the belt, and is pressed by a pressure roll in a non-stretched state thereby to be driven to rotate. The fixing belt can be warmed up in a shorter time than the roll type member, because it has a thin walled heat resistant resin as a base layer, and thermal capacity is smaller as compared with the roll type member. Moreover, in the fixing belt of the non-stretched type, a contact area with respect to other members can be minimized, and heat transfer to the other members can be decreased. Consequently, more efficient warming up can be performed.
The fixing devices in which the fixing belt is driven to rotate in a non-stretched state are disclosed in JP-A-07-281461 and JP-A-2003-223064. In these fixing devices, a guide member and a heat source are provided inside the fixing belt which is in an endless state, so that the fixing belt is rotatably supported and heated from the inside. A pressure roll is contacted with an outer peripheral face of the fixing belt, and pressed onto the guide member thereby to form a fixing nip. When this pressure roll is driven to rotate, the fixing belt will be rotated with a friction force in the fixing nip. Then, the recording medium carrying the unfixed toner image will be brought into contact with the fixing belt while passing through the fixing nip, and then, heated and pressurized between the guide member and the pressure roll, whereby the toner image will be fixed on the recording medium.
Moreover, described in JP-A-2003-84591 is a fixing device in which a fixing belt in a substantially non-stretched state is heated by an electromagnetic induction heater. Specifically, the fixing belt is provided with an electrically conductive layer which is heated by induced current, along its peripheral face, and an exciting coil is arranged so as to be opposed to the outer peripheral face of the fixing belt. Variable magnetic field induced by this exciting coil induces eddy current in the electrically conductive layer of the fixing belt to heat it up. Moreover, in this fixing device, a center part of the fixing belt is in a state where almost no tension is exerted in a circumferential direction, but in side edge parts thereof, a guide member is contacted with the inner peripheral face of the fixing belt in the circumferential direction, so that deformation and swing of the fixing belt may be restrained.
On the other hand, disclosed in JP-A-2003-122149 and JP-A-07-281549 are fixing devices in which rotation driving force is given from side edge parts of a fixing belt in a non-stretched state. In the fixing device disclosed in JP-A-2003-122149, both side edges of the fixing belt (a fixing film) in an endless state have concaves and convexes in a circumferential direction and rotation receiving members are engaged with these concaves. Driving force is transmitted to these rotation receiving members thereby to drive the fixing belt. In the fixing device disclosed in JP-A-07-281549, the fixing belt is provided with projections in a cogwheel shape in side edge parts thereof, and a driving gear is meshed with the projections to give the driving force to the fixing belt. There is further disclosed, in JP-A-07-281549, the fixing device in which the fixing belt is provided with engaging holes which are circumferentially arranged in the side edge parts thereof, and adapted to be engaged with engaging projections formed on a peripheral face of the roll type member thereby to transmit the driving force.
However, the fixing device employing the above described fixing belt in a non-stretched state has had the following problems to be solved.
In the fixing devices disclosed in the above described JP-A-07-281461, JP-A-2003-223064 and JP-A-2003-84591, the rotation driving force of the fixing belt is transmitted by friction force from the pressure roll which is pressure contacted with the outer peripheral face of the fixing belt. Moreover, the guide member is pressure contacted with the inner peripheral face of the fixing belt, and the fixing belt performs sliding movement. Accordingly, in an initial period of using the device, a sliding face thereof is in good condition, and the fixing belt can be driven to rotate with no problem in association with the rotation of the pressure roll. However, as the device is continuously used, interior sliding resistance will rise, and rotation speed of the fixing belt may be sometimes delayed with respect to rotation of the pressure roll. This is due to a slip which has happened on a contact face between the fixing belt and the pressure roll, and will be a cause for instability of an image to be fixed on the recording medium, or occurrence of paper wrinkles in the recording medium.
Moreover, as temperature of the pressure roll rises, thermal expansion occurs in material which the peripheral face of the pressure roll is formed of, and a length of the peripheral face is made longer. When this pressure roll is driven to rotate at a determined rotation number, driving speed of the fixing belt will vary according to the temperature of the pressure roll. Then, it is concerned that unbalance in speed may happen with respect to a transfer step in which the recording medium is conveyed always at a constant speed, and instability of the image or wrinkle of paper may occur.
Moreover, in the fixing devices disclosed in JP-A-07-281461 and JP-A-2003-223064, an orbit of the fixing belt is not sufficiently restrained except the nip part, and a swing of the belt or a sway of the belt in a direction perpendicular to the peripheral face may happen, while the belt is driven. Such positional instability of the belt may sometimes cause variations in distance between the belt and the exciting coil which is opposed to the belt, and unevenness in heating temperature may occur. As the results, the fixed image may be damaged.
On the other hand, in the device disclosed in JP-A-2003-84591, the fixing belt hardly swings because a shape of the fixing belt is restrained in the side edge parts thereof. However, the guide member provided on the inner face scrapes the inner peripheral face of the fixing belt while sliding along the circumferential direction, which increases friction. Moreover, in case where the guide member on the inner face has been made larger in diameter to apply tension to the fixing belt from inside at both end parts thereof, larger frictional force will be exerted, and the rotation speed of the fixing belt will be more liable to be lowered.
Further, in the devices disclosed in JP-A-2003-122149 and JP-A-07-281549, there is such an anxiety that necessity of adding delicate works on a thin film metal sleeve or an endless belt which is used as the fixing belt may happen, or stress may be concentrated on end parts of the thin film metal sleeve and engaging holes in the endless belt, and breakage or fatigue failure may arise from these parts.